The present invention relates to a device for detecting an air-fuel ratio of an air-fuel mixture over a wide range from below the stoichiometric ratio (rich) for the mixture to above the stoichiometric ratio (lean) for the mixture by exposing a probe or a sensing element to exhaust gases resulting from combustion of the mixture.
In automobiles, it is necessary to detect an air-fuel ratio of a mixture supplied to an internal combustion engine for controlling the supply of fuel to the engine so as to adjust the actual air-fuel ratio to a target value. Common practice to detect the air-fuel ratio is to expose an oxygen sensing element, viz., an oxygen sensor, to exhaust gases discharged by the internal combustion engine and measure oxygen partial pressure in the exhaust gases.
Japanese patent application primary publication No. 57-76450 discloses a device for detecting an air-fuel ratio of an air-fuel mixture having an air-fuel ratio above the stoichiometric ratio for the mixture by measuring an oxygen partial pressure in exhaust gases resulting from the combustion of the mixture by exposing an oxygen sensing element to the exhaust gases. This known device is further described referring to FIG. 1. As illustrated in FIG. 1, the oxygen sensing element comprises an oxygen ion-conductive solid eletrolyte 1 having a measurement electrode layer 2 on one side thereof and a reference electrode layer 3 on the other side thereof. A DC voltage is applied between the electrode layers 2 and 3 to cause an electric current I.sub.s to flow through the solid electrolyte 1 from the electrode 3 to the electrode 2. For restricting inflow of oxygen to the measurement electrode 2, a porous coating layer 4 covers the electrode 2. Another coating layer 5 covers and protects the other electrode layer 3. The inflow of the electric current I.sub.s causes oxygen ions O.sup.2- to migrate from the electrode layer 2 to the electrode layer 3. As a result, a reference oxygen partial pressure Pa develops at the reference electrode layer 3 and an oxygen partial pressure Pb, viz., an oxygen partial pressure in the exhaust gases, develops at the measurement electrode 2. An electromotive force E produced by the sensing element may be expressed by Nernst's equation as follows: EQU E=(RT/4F) ln(Pa/Pb) (1)
where: R is the gas constant, T the absolute temperature, and F the Faraday constant.
This electromotive force E may be measured and taken out in terms of an output Vs of the sensing element. The output Vs exibits different voltage versus .lambda. characteristics for different magnitudes of the electric current I.sub.s as shown in FIG. 2 where .lambda.=(actual air-fuel ratio)/(the stoichiometric ratio). As will be readily understood from FIG. 2, since, if the electric current I.sub.s is kept constant, the output Vs varies versus air-fuel ratio within a narrow range of .lambda., an actual air-fuel ratio within the narrow range can be detected by the sensor output Vs. This. however, is not practical for detection of air-fuel ratio over a wide range. To overcome this problem, it is proposed to keep the voltage Vs at a target value Va (see FIG. 2) and measure an electric current I.sub.s which is variable in proportion to variation in .lambda. over a wide range as shown by a fully drawn curve in FIG. 3 as long as the air-fuel ratio is above the stoichiometric ratio (.lambda.&gt;1).
This known device, however, is not suitable for detecting an air-fuel ratio of a rich mixture (.lambda.&lt;1) because, as will be understood from FIG. 3, the current I.sub.s increases again as the mixture becomes rich as shown by a broken line curve. This characteristic exibited by the current I.sub.s when the air-fuel ratio is below the stoichiometric ratio is derived from the fact that an equilibrium state with the exhaust gases resulting from combustion of a rich air-fuel mixture is not accomplished so that oxygen ions within the solid electrolyte are only diffused into the ambient exhaust gas environment in the form of oxygen molecules because the content of oxygen in the exhaust gases is almost zero. This explains why the migration of oxygen ions increase and thus the electric current I.sub.s increases as the air-fuel ratio shifts to the rich side beyond the stoichiometric ratio.
As a result, with the same measured magnitude in the electric current I.sub.s, a single air-fuel ratio cannot be identified because two air-fuel ratio values are present within a range near the stoichiometric ratio. Thus, it is impossible to identify the actual air-fuel ratio by relying on the measurement result of the electric current I.sub.s only. In other words, the use of this known device is confined to detecting an air-fuel ratio above the stoichiometric ratio. Besides, since both of the electrode layers 2 and 3 are exposed to the exhaust gases, the electrodes 2 and 3 are deteriorated at a fast rate. The solid electrolyte is deteriorated, too, when the sensing element is used for a long time to detect a rich air-fuel ratio because a material ZrO.sub.2 which constitutes the solid electrolyte 1 is decomposed into ions, oxygen ions of which are diffused into the exhaust gases. Thus, the output characteristic of the sensing element (I.sub.s versus A/F characteristic) varies for a time (age) and the endurability is not satisfactory.